1. Field of the Invention
The present invention relates to a technology for encoding a stereo signal to compress an audio signal.
2. Description of the Related Art
Conventionally, as a scheme of encoding a frequency spectrum obtained by orthogonally transforming an audio signal such as those of voice and music, an advanced audio coding (AAC) that is an audio standard of ISO/IEC 13818-7 has been used. The AAC is applied to a surface digital radio broadcasting, and a mid-side (MS) stereo encoding is further applied to improve efficiency of compression of the stereo signal.
FIG. 12 is a schematic for illustrating an encoding procedure in the MS stereo encoding. An MS stereo encoding apparatus 1200 shown in FIG. 12 first orthogonally transforms a left channel audio signal (L) by an L-orthogonally transforming unit 1201 and orthogonally transforms a right channel audio signal (R) by an R-orthogonally transforming unit 1202. The L and R after the transformation are input into an MS stereo transforming unit 1203 and the MS stereo transforming unit 1203 generates respectively a sum signal M (M=(L+R)/2) and a difference signal S (S=(L−R)/2) from the input L and R. The sum signal M is encoded by a sum signal quantizer 1204 (code word 1). The difference signal S is encoded by a difference signal quantizer 1205 (code word 2).
In MS stereo encoding, in the MS stereo transforming unit 1203, when L and R are highly correlated with each other, that is, L and R are highly similar to each other, the electric power of the difference signal S is smaller than that of the sum signal M. Therefore, the efficiency of the encoding can be improved by decreasing the number of encoding bits of the difference signal S and increasing the number of encoding bits of the sum signal M.
In addition to the transformation by the MS stereo encoding, as a method of improving the efficiency of encoding, for example, Japanese Patent Application Laid-Open Publication No. 2001-255892 discloses a technique that transforms adaptively a difference signal into a monaural state. FIG. 13 is schematic for explaining an adaptive transformation into the monaural state. Charts 1310 and 1320 show the spectrums of audio signals L and R. Charts 1330 and 1340 show the spectrums of a sum signal M and a difference signal S generated using the L and R. A spectrum 1311 of the L and a spectrum 1321 of the R are transformed respectively into a spectrum 1331 of the sum signal M and a spectrum 1341 of the difference signal S.
In the transformation from the L and R into the sum signal M and the difference signal S, a signal at a frequency “f” is noted. In the monaural transformation, similarity between the L and the R is obtained, and when the similarity between the L and the R is high, the difference signal S is silenced or is deformed into a signal having small amplitude. When the similarity between the L and the R is high, the number of bits of the difference signal S is decreased to zero because the difference signal S becomes S=(L−R)/2≈0. That is, for the spectrum 1341 representing the difference signal S, the signal at the frequency f becomes zero and the bits for this signal is allocated to the signal at the frequency f of the spectrum 1331 representing the sum signal M. Therefore, the number of bits of the sum signal M is increased and distortion of the audio signal associated with the quantization can be reduced.
However, in the surface digital radio broadcasting, the bit rate allocated to sound is very low as 32 kilo bits per second (kbps) to 64 kbps to realize high-quality sound (music) at the quality level of a CD and video images at around 330 kbps in total. Therefore, in the conventional MS stereo encoding, sound quality is degraded due to shortage of the number of quantization bits.
If the adaptive transformation into the monaural state is applied, in a band of the difference signal S being zero, which is a band that has been transformed into the monaural state, the number of quantization bits of the difference signal S can be decreased. However, in a band that can not be transformed into the monaural state, the number of quantization bits of the difference signal S can not be decreased. Therefore, sufficient sound quality can not be obtained under the condition of a low bit rate.